The present invention relates to an optical recording medium having at least an optical absorptive layer and a light reflective layer on a transparent substrate, and a method for recording information on such an optical information recording medium.
As an optical information recording medium capable of recording data by irradiation of a laser beam, an optical information recording medium is known which has a recording layer composed of e.g. a layer of a metal such as Te, Bi or Mn or a layer of a dye such as cyanine, mellocyanine or phthalocyanine and which is designed to record data in such a manner that when irradiated with a laser beam, the recording layer is deformed, sublimed, evaporated or modified to form pits. Further, an optical information recording medium having a recording layer of a dye and a reflective layer of a metal formed on a light transmitting substrate is known.
On the other hand, optical information recording media of so-called ROM (read only memory) type wherein data are already recorded, and no further data are recordable or the recorded data are no longer erasable or rewritable, are widely practically employed in the sound recording and information treating fields. Optical information recording media of this type have no such a recording layer as described above. Pits corresponding to data to be reproduced are already formed on a substrate of e.g. polycarbonate by e.g. press molding, a reflective layer of a metal such as Au, Ag, Cu or Al is formed thereon, and a protective layer is further provided thereon.
The most typical optical information recording medium of this ROM type is a compact disc so-called CD which is widely used in the sound recording and information treating fields. The specification for read-out signals for this CD is standardized, and the reproducing or play back apparatus for CD in accordance with the standards is widely available as compact disc players (CD players).
To satisfy the CD standards, it is required that the reflectance is at least 70%, and with respect to the degree of modulation of the read-out signals, I11/Itop is at least 0.6, and I3/Itop is from 0.3 to 0.7.
The above-mentioned writable optical information recording media employ a laser beam in the same manner as CD. Therefore, it is strongly desired for such media to follow the same standards as the widely prevailing standards for CD.
However, conventional writable optical information recording media have a recording layer which is not present in CD, and they are designed to form pits in the recording layer and not in the substrate. Further, in some cases, a space is provided to facilitate the formation of pits in this recording layer, and the reflectance of a laser beam is low as compared with CD. Consequently, the read-out signals are naturally different from those of CD. Therefore, it has been difficult for the conventional optical information recording media to meet the above-mentioned standards for CD. Particularly, the reflectance and the degree of modulation of the read-out signals used to be too low to meet the standards. Therefore, it has been difficult to provide a writable optical information recording medium which satisfies the CD standards.
Therefore, it is an object of the present invention to provide a writable optical information recording medium which has high reflectance and which is capable of providing read-out signals having a high degree of modulation to meet the CD standards.
The present invention provides an optical information recording medium comprising a light transmitting substrate, a light absorptive layer containing at least one light absorbing substance formed on the substrate and a light reflective layer made of a metal film formed on the light absorptive layer, wherein an optical parameter represented by xcfx81=nabsdabs/xcex wherein nabs is the real part of the complex refractive index of the light absorptive layer, dabs is the average thickness of the light absorptive layer and xcex is the wavelength of a reading laser beam, is 0.6 less than xcfx81 less than 1.6. The imaginary part kabs of the complex refractive index of the light absorptive layer is preferably at most 0.2.
The present invention also provides a method for recording information on an optical information recording medium, wherein the above optical information recording medium is used, and the recording is conducted by irradiating a laser beam from the substrate side of the medium.